Isothermal sections of the phase diagrams for the systems Ln-Pd-O (Ln = Dy, Ho) at 1223 K have been established by isothermal equilibration of samples at high temperature, and phase identification after quenching by optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray powder diffraction (XRPD). The binary oxide PdO was not stable at 1223 K. The oxide phase, Ln(2)O(3) was stable along the binary Ln-O. Only one ternary oxide Ln(2)Pd(2)O(5) was identified in the two systems under investigation. Liquid alloys, the six intermetallic compounds (Ln(3)Pd(2), LnPd, Ln(3)Pd(4), Ln(2)Pd(3), Ln(10)Pd(21) and LnPd(3)) and Pd-rich solid solution were found to be in equilibrium with Ln(2)O(3). Based on the phase relations, two solid-state cells were designed to measure the Gibbs energies of formation of the ternary oxides. An advanced version of the solid-state cell incorporating a buffer electrode was used for high-temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace hole conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. Yttria-stabilized zirconia (YSZ) was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. Electromotive force (emf) measurements, were conducted in the temperature range from 900 to 1250 K. The standard Gibbs energy of formation (Delta(f (ox)) Gdegrees, J mol(-1)) of the interoxide compounds from their component binary oxides Ln(2)O(3) with C-type structure and PdO can be represented by the following equations: Dy2Pd2O5: Delta(f(ox)) Gdegrees = -33 735 - 1.50T (+/-770) Ho2Pd2O5: Delta(f(ox))Gdegrees = -23 975 - 0.99T (+/-740) Based on the thermodynamic information, isothermal chemical potential diagrams for the system Ln-Pd-O (Ln = Dy, Ho) are developed. (C) 2002 Elsevier Science B.V. All rights reserved.